Evolved Packet System (EPS) is the Evolved 3GPP Packet Switched Domain and includes Evolved Packet Core (EPC) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
The EPC architecture is defined in 3GPP TS 23.401, which provides definitions of the PGW (PDN Gateway), SGW (Serving Gateway), PCRF (Policy and Charging Rules Function), MME (Mobility Management Entity), and mobile device (User Equipment, UE). The Long-Term Evolution (LTE) radio access, E-UTRAN, includes one more eNBs (also referred to as base stations). FIG. 1 illustrates non-roaming EPC architecture for 3GPP accesses.
FIG. 2 illustrates an overall E-UTRAN architecture and is further defined, for example, in 3GPP TS 36.300. The E-UTRAN includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY or Packet Data Convergence Protocol/Radio Link Control/Medium Access Control/Physical Layer) and control plane (Radio Resource Control, or RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME interface and to the Serving Gateway (S-GW) by means of the S1-U interface.
Portions of the EPC Control Plane (CP) and User Plane (UP) architectures are shown in FIGS. 3 and 4. FIG. 3 illustrates the EPC Control Plane protocol architecture, and FIG. 4 illustrates the EPC User Plane protocol architecture.
LTE Dual Connectivity (DC) is standardized in 3GPP Rel-12 to enable UEs to send and receive data on multiple carriers at the same time (e.g., multiple TX/RX). As described in 3GPP TS 36.300, E-UTRAN supports DC operation whereby a multiple Rx/Tx UE in RRC_CONNECTED is configured to utilize radio resources provided by two distinct schedulers, located in two eNBs connected via a non-ideal backhaul over the X2 interface (see 3GPP TRs 36.842 and 36.932). The overall E-UTRAN architecture depicted in FIG. 2 is applicable for DC as well. eNBs involved in DC for a certain UE may assume two different roles: an eNB may either act as an MeNB (Master eNB) or as an SeNB (Secondary eNB). In DC, a UE is connected to one MeNB and one SeNB.
In DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three bearer types are MCG (Master Cell Group) bearer, SCG (Secondary Cell Group) bearer, and split bearer. Those three bearer types are shown in FIG. 5, which illustrates Radio Protocol Architecture for Dual Connectivity. RRC is managed in a MeNB, and SRBs (Signaling Radio Bearers) are always configured as MCG bearer type and therefore only use the radio resources of the MeNB. Note that DC can also be described as having at least one bearer configured to use radio resources provided by the SeNB.
Inter-eNB control plane signaling for DC is performed by means of X2 interface signaling. Control plane signaling towards the MME is performed by means of S1 interface signaling. There is only one S1-MME connection per DC UE between the MeNB and the MME. Each eNB should be able to handle UEs independently, i.e., provide the PCell to some UEs while providing SCell(s) for SCG to others. Each eNB involved in DC for a certain UE controls its radio resources and is primarily responsible for allocating radio resources of its cells. Respective coordination between MeNB and SeNB is performed by means of X2 interface signaling. FIG. 6 illustrates CP connectivity of eNBs involved in DC for a particular UE. The S1-MME is terminated in MeNB, and the MeNB and the SeNB are interconnected via X2-C.
For DC, two different user plane architectures are allowed: one in which the S1-U only terminates in the MeNB and the user plane data is transferred from MeNB to SeNB using the X2-U; and a second architecture where the S1-U can terminate in the SeNB. FIG. 7 illustrates different UP connectivity options of eNBs involved in DC for a certain UE. Different bearer options can be configured with different user plane architectures. UP connectivity depends on the bearer option configured.
For MCG bearers, the S1-U connection for the corresponding bearer(s) to the S-GW is terminated in the MeNB. The SeNB is not involved in the transport of user plane data for this type of bearer(s) over the Uu. For split bearers, the S1-U connection to the S-GW is terminated in the MeNB. PDCP data is transferred between the MeNB and the SeNB via X2-U. The SeNB and MeNB are involved in transmitting data of this bearer type over the Uu. For SCG bearers, the SeNB is directly connected with the S-GW via S1-U. The MeNB is not involved in the transport of user plane data for this type of bearer over the Uu. Note that if only MCG and split bearers are configured, there is no S1-U termination in the SeNB.
The SeNB Addition procedure is initiated by the MeNB and is used to establish a UE context at the SeNB in order to provide radio resources from the SeNB to the UE. This procedure is used to add at least the first cell (PSCell) of the SCG. FIG. 8 shows this SeNB Addition procedure.
Multi-connectivity can also be envisioned as an important feature for fifth-generation (5G) RAN architectures standardized by 3GPP. FIG. 9 illustrates a high-level view of the 5G network architecture, consisting of a Next Generation RAN (NG-RAN) and a 5G Core (5GC). The NG-RAN can comprise a set of gNodeB's (gNBs) connected to the 5GC via one or more NG interfaces, whereas the gNBs can be connected to each other via one or more Xn interfaces. Each of the gNBs can support frequency division duplexing (FDD), time division duplexing (TDD), or a combination thereof.
The 5G RAN logical nodes shown in FIG. 9 (and described in TR38.801 v1.2.0) include a Central Unit (CU) and one or more Distributed Units (DU). The CU is a logical node that is a centralized unit that hosts high layer protocols and includes a number of gNB functions, including controlling the operation of DUs. A DU is a decentralized logical node that hosts lower layer protocols and can include, depending on the functional split option, various subsets of the gNB functions. As used herein, the terms “central unit” and “centralized unit” are used interchangeably, and the terms “distributed unit” and “decentralized unit” are used interchangeability.
A CU may host protocols such as RRC and PDCP, while a DU may host protocols such as RLC, MAC and PHY. Other variants of protocol distributions between CU and DU exist, such as hosting the RRC, PDCP and part of the RLC protocol in the CU (e.g., Automatic Retransmission Request (ARQ) function), while hosting the remaining parts of the RLC protocol in the DU, together with MAC and PHY. In some exemplary embodiments, the CU is assumed to host RRC and PDCP, where PDCP is assumed to handle both UP traffic and CP traffic. Nevertheless, other exemplary embodiments may utilize other protocol splits that by hosting certain protocols in the CU and certain others in the DU. Exemplary embodiments can also locate centralized control plane protocols (e.g., PDCP-C and RRC) in a different CU with respect to the centralized user plane protocols (e.g., PDCP-U).
In the architecture identified by CUs and DUs, DC can be achieved by means of allowing a UE to connect to multiple DUs served by the same CU or by allowing a UE to connect to multiple DUs served by different CUs.
As illustrated in FIG. 9, a gNB can include a gNB-CU connected to one or more gNB-DUs via respective F1 interfaces, all of which are described hereinafter in greater detail. In the NG-RAN architecture, however, a gNB-DU can be connected to only a single gNB-CU.
The NG-RAN is layered into a Radio Network Layer (RNL) and a Transport Network Layer (TNL). The NG-RAN architecture, i.e., the NG-RAN logical nodes and interfaces between them, is defined as part of the RNL. For each NG-RAN interface (NG, Xn, F1) the related TNL protocol and the functionality are specified. The TNL provides services for user plane transport and signaling transport. In NG-Flex configuration, each gNB is connected to all 5GC nodes within a pool area. The pool area is defined in 3GPP TS 23.501. If security protection for control plane and user plane data on TNL of NG-RAN interfaces has to be supported, NDS/IP (3GPP TS 33.401) shall be applied.
3GPP Tdoc R3-173235 discloses mechanisms to achieve fast retransmission of lost Protocol Data Units (PDUs) in case of radio link outage. R3-173235 focuses on the case where a DU communicates to the CU via an F1 interface, i.e., within a single gNB. Further, the document focuses on the case where a radio blockage event occurs for a limited amount of time, after which the radio link is back to good radio quality. The document mentions a possible case where the radio link in question becomes unstable, however no solutions are described to address this case.
As mentioned above, multi-connectivity (e.g., DC) is envisioned as an important feature to be supported in RAN 5G architectures. In this context, DC support includes establishing master (MN) and secondary nodes (SNs) and distributing UP traffic to the MN and SNs according to optimal, preferred, and/or desirable traffic and radio resource management techniques. CP traffic is assumed to terminate in one node only, i.e. the MN. FIGS. 10 and 11 show the protocol and interfaces involved in DC, as described in 3GPP TS 38.300v0.6.0. FIG. 10 shows that the Master gNB (MgNB) can forward PDCP bearer traffic to a Secondary gNB (SgNB), while FIG. 11 shows the case where the SgNB can forward PDCP bearer traffic to the MgNB. In some exemplary embodiments, the MgNB and/or SgNB can be subject to the RAN split architecture (e.g., CU and DU) discussed above.
Furthermore, multi-RAT dual connectivity (MR-DC) can also be envisioned as an important feature in 5G RAN architectures. When MR-DC is applied, the MN can anchor the control plane towards the CN, while the SN can provide control and user plane resources to the UE via coordination with the MN. This is illustrated in FIG. 12, which is extracted from 3GPP TS 37.340. Within the scope of MR-DC, various user plane/bearer type solutions are possible, as seen in FIG. 13, also from TS 37.340.
Due to variable radio channel quality between a UE and an eNB (4G) or a gNB (5G), interruptions or blockages in the data throughput over the radio link can occur. Removing a blocked radio link frees resources that can be used to server other UEs or users. The removal decision must also take into account the likelihood that the blocked radio link will return to normal performance within a short time window. If this occurs, a maintained link could be reused without the need to set it up again from scratch, which also saves resources.